The heavy chain switch is mediated by recombination event between switch (S) regions associated with the donor (usually mu) and recipient (gamma (g), epsilon (e), or alpha (a)) heavy chain genes. Aberrant switch recombination events lead to the chromosomal translocations found in Burkitt's lymphoma, AIDS-associated lymphoma, and perhaps other lymphoid tumors. Hence, understanding the heavy chain switch is important for both improving immune responses to pathogens and to our understanding of the genesis of lymphoid tumors. Switch recombination is preceded by germline transcription through the S region and the constant region. Although switch recombination is understood to be a DNA deletion beginning and ending in S regions, almost nothing else is known about its mechanism. Switch recombination is also regulated in that it is directed by extracellular signals to one, or sometimes, two heavy chain genes. Cis-acting elements in the heavy chain locus must play a role in gene-specific switch recombination. To understand the mechanism of switch recombination, and to understand the cis elements that are important to both regulation and the recombination event itself, we have applied new technology to modify genes in a cloned version of the murine heavy chain locus. In the proposed experiments we will modify a specific heavy chain gene on a bacterial artificial chromosome with an assembled variable region and the entire heavy chain constant region locus, and test expression of various heavy chain genes in transgenic mice. We will test the role of a specific protein binding site or promoter regions, in general, for murine gamma heavy chain genes in both germline transcription and switch recombination (Aims 1 and 2). We will test how deletion or modification of the switch region changes switch recombination (Aim 3). We will examine the role of the orientation of switch sequences, and hence the role of the unusual complexes between germline transcripts and switch region DNA, in Aim 4.